1. Field of the Invention
The present invention relates to the design of semiconductor light-emitting devices. More specifically, the present invention relates to novel device structures which facilitate more efficient light emission.
2. Related Art
Solid-state lighting is expected to be the illumination wave of the future. High-brightness light-emitting diodes (HB-LEDs) are beginning to penetrate an increasing number of applications, especially as the light source for display devices and as light-bulb replacement for conventional lighting. Higher light-emission efficiency, like for conventional incandescent or florescent lights, remains a challenge for LED designers.
An LED typically produces light from an active region which is situated between a positively-doped cladding layer (p-type cladding layer) and negatively-doped cladding layer (n-type cladding layer). When the LED is forward-biased, the carriers, which include holes from the p-type cladding layer and electrons from the n-type cladding layer, recombine in the active region. For direct band-gap materials, this recombination process releases energy in the form of photons, or light, whose wavelength corresponds to the energy band-gap of the material in the active region.
To improve the light-emission efficiency, it is critically important for emitted light to leave the device promptly, so that the light is not absorbed by the inactive materials in the device. Unlike laser devices, wherein emitted light is guided and propagates in a common, well-defined direction, the light emitted in an LED propagates omni-directionally. Hence, a portion of the light is reflected off certain internal surfaces, absorbed, or is obstructed by non-transparent materials within the device.
The light-obstruction issue is particularly pronounced in an LED with a vertical-electrode structure, that is, the entire device is situated between an upper and lower electrodes. An electrode is typically made of metal, which is non-transparent to visible light. When a significant overlap exists between the upper and lower electrodes, the electrodes can obstruct a substantial amount of vertically emitted light. In addition, more carriers recombine in the active region which coincides with the overlapped area, since this region typically is part of a low-resistance path between the electrodes. The concentration of carriers in this undesirable location further exacerbates the vertical-light obstruction problem.
Hence, what is needed is an LED structure that mitigates the vertical-light obstruction problem and a method for fabricating such a structure.